Today's conventional diesel engines produce nitrogen oxides (NOx), which play a role in forming photochemical smog. Yet diesel engines are so durable, reliable, and efficient, it is important to keep them as viable options for transportation.
Increasingly strict regulations have prompted research for new ways of controlling the polluting emissions from diesel engines without compromising fuel economy. In the United States, Environmental Protection Agency rules require cleaner diesel fuel (lower sulfur content) and more stringent engine standards (reducing particulate matter and nitrogen oxide emissions).
Researchers have demonstrated that nitrogen oxide emissions can be reduced by exhaust recirculation in both gasoline and diesel engines. However, only a limited amount of exhaust can be recirculated without reducing power output and fuel economy. Recirculated soot particles may also cause wear in diesel engines. Other methods for NOx reduction that are being studied are fuel-water emulsion, selective catalytic reduction with ammonia or urea, lean NOx catalysts, and NOx adsorbers.
NOx adsorbers (NOx traps) are a promising development as results show that NOx adsorber systems are less constrained by operational temperatures than lean NOx catalysts. NOx traps adsorb and store NOx under lean conditions. A typical approach is to speed up the conversion of nitric oxide (NO) to nitrogen dioxide (NO2) using an oxidation catalyst so that NO2 can be readily stored as nitrate on alkaline earth oxides. A brief return to stoichiometric or rich operation for one or two seconds is enough to desorb the stored NOx and provide the conditions of a conventional three-way catalyst mounted downstream to destroy NOx.
One design for using NOx adsorber technology is referred to as a “dual leg” or “dual path” design. In these systems, the exhaust path splits into two paths, with a NOx adsorber on each new path. Typically, one path receives most of the exhaust flow while the other path is in regeneration mode. After regeneration, that path begins to receive most of the exhaust flow while the other path regenerates. The paths continually switch modes in this manner.